"The flowering cherries, the pomegranate trees -- the novel suffuses the reader with a sense of old Japan." - Susan Salter Reynolds, The Los Angeles Times

"Kokoro is perhaps his masterpiece: a slice of nineteenth-century naturalism purified and transmuted so that its final effect is poetic. (...) Soseki's Kokoro has an unforgettable atmosphere of the loneliness and mysteriousness of man's relationship with man." - Anthony Thwaite, Times Literary Supplement

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For much of Kokoro the novel seems singularly unrevealing.
The novel is presented in three parts, the first two narrated by an anonymous young man, the last in the form of a letter that an older man he looks upon as his mentor (and whom he calls Sensei) writes to him.
The chapters are extremely short (there are a total of 110), artificially chopping up the narrative into two-page bits, which helps prevent much of a sense of intimacy developing.
Many details -- from names to what the characters study at university to the substance of their conversations -- remain unspecified or vague.
Throughout the book, an unwillingness to bring up significant matters leads to misunderstanding and disaster, and even the closing sentences of the novel are an appeal to discretion:

I want her told nothing. [...] I therefore ask that you keep all this to yourself, a secret intended for your eyes alone.

The reason the story does work -- and ultimately proves affecting -- is that Sensei does reveal something in the end, explaining what happened in his past that so marked his life -- and it is presented in such a way, so carefully built up, that the reader is made to feel s/he has been let in on "a secret intended for your eyes alone".
Oh, yeah, and Sensei offs himself, which adds to the (melo)drama.
The narrator begins his story describing how he is drawn to Sensei, whom he first encounters on the beach at Kamakura while vacationing.
He doesn't completely force himself on the old man, but certainly seeks out his friendship -- and Sensei is willing to tolerate it.
Obviously, Sensei sees himself in the young student too, and understands that the narrator is seeking some companionship and guidance that he can't find among those his own age:

"You came to me because of some lack you sensed, didn't you ?"

The narrator often goes to visit Sensei, and relates some of what they talk about, but usually only the same generalities.
There are hints of there being more to Sensei's story (which includes frequent pilgrimages to a cemetery), but the narrator doesn't go into it: "I will say nothing of the tragedy yet" is about as far as he'll go.
Nevertheless, the mentions and hints of some dark secret -- and Sensei's insistence that no one is to be truly trusted ("In any event, you mustn't trust me too much") -- are liberally dosed in the story, in order to keep the reader interested.
Otherwise: fairly little happens, and fairly little is revealed in the first half of the book.
Over the course of the first two sections the narrator's father grows increasingly ill.
The narrator completes his studies in Tokyo and, still unsure of what to do with himself, also spends more time with his family.
They, in turn, hope that Sensei might be able to get him a decent position -- though in fact Sensei is useless in this regard, having largely withdrawn from the world and having no reputation whatsoever:

Sensei's name was quite unknown in the world.
I seemed to be the only person who was in a position to really respect him for his learning and ideas.

(Oddly, however, none of his learning and few of his ideas are related by the narrator, who sticks to the very vague stuff in his account.)
The narrator's father's health continues to decline, until he is near death.
As if one crisis involving a father-figure were not enough, that's when the narrator gets a fat letter from Sensei -- and rushes off the Tokyo to try to save him.
(We actually never learn whether or not he manages to do so, but given that he did not immediately
read the letter, it seems highly unlikely.)
The third section of the book, called 'Sensei's Testament', consists entirely of the letter, a suicide note that covers about half the novel -- and tells an entirely different story, as Sensei revisits the time when he was the narrator's age.
Cheated out of most of his inheritance while at university, Sensei became a mistrustful soul.
Even worse, however, he became involved in a sort of love-triangle, which ended very, very badly.
One of the reasons things went so wrong in his youth was that Sensei wouldn't speak up, or act.
Society's expectations and rules (and the Japanese character) -- and his own weakness -- hem him in.
Typically:

Yet though my decision was made, I still held back from day to day.
I know this makes me seem terribly irresolute -- I don't mind that it does.
But my inability did not stem from lack of willpower.

Maybe not -- but he still makes a mess of things by holding back.
And he continued to hold back throughout his life, even after the disaster, leading to him becoming the man the narrator came to know.
Specifically, he never opened up to his wife -- and remains torn apart by that to the end.
Yet even in death he can't reveal what weighs upon him so, his dying wish being that she continue to be spared the truth.
Sensei presumably means to teach the narrator a lesson (including the practical one of getting his father to do some estate-planning before it is too late, something Sensei repeatedly urges on the young man) -- but he also writes of having wanted to tell his story in person (being prevented from doing so by the narrator's obligation to be with his father), suggesting he was looking absolution and a savior -- someone who could talk him down from the ledge.
Intriguingly, the novel ends with the end of Sensei's 'testament'; there is no confirmation of his actual death, or description of what the narrator found when he arrived in Tokyo.
Kokoro is rather longwinded -- or rather, it is slow to get wound up.
The narrator's part is secondary, a base that Sōseki needs to make Sensei's account -- the heart and meat of the novel -- more effective; it's not particularly compelling -- indeed, fairly boring (and feels rather underdeveloped).
But the naïve young man, at an early crossroads as his father-figures are removed from his life (conveniently with that mother of all father-figures, the Japanese emperor, also dying at the same time), does have something of the younger Sensei in him, and that contrast and comparison also flourishes when Sensei finally tells his story.
A commentary on the Japanese character in those times, a Bildungsroman, and a tragic love story, Kokoro is a rich novel that doesn't quite work along traditional Western lines: no European author of the same period could have sustained so much avoidance for so long.
It can't be read like a European novel of those times, either, but while the first two sections can be, in part, a confounding slog it comes very much into its own in the long final section.
Unusual, but ultimately worthwhile.